(i) Field of the Invention
The present invention relates to a BALUN transformer core material, a BALUN transformer core which is obtained by pressing and sintering the BALUN transformer core material, and a BALUN transformer which can be operated in a frequency band of 300 MHz or more.
(ii) Description of the Related Art
The BALUN transformer (Balance to Unbalance transformer: hereinafter abbreviated as BALUN) is a conversion component for smoothly connecting an unbalance circuit and a balance circuit, and used for various electronic apparatus, e.g., an antenna in a communication circuit and the like. Additionally, since the electronic apparatus have been made compact and operated at a high frequency in recent years, an essential countermeasure needs to be taken on constituent components of the electronic apparatus. The BALUN transformer is not an exception.
The BALUN transformer usually has a structure in which a conductor wire constituted of a metal is wound double around a toroidal or solenoid BALUN transformer core to form a winding (bifilar winding). As the BALUN transformer core, there is used a BALUN transformer core which is obtained by pressing and sintering a BALUN transformer core material having a high magnetic permeability, e.g., a spinel Ni--Cu--Zn ferrite as an oxide magnetic material. The spinel Ni--Cu--Zn ferrite has a relatively high magnetic permeability and specific resistance, and is not only used as the BALUN transformer core material but also frequently used in a high-frequency coil or other various electronic components.
However, the magnetic permeability of the spinel ferrite including the Ni--Cu--Zn ferrite has a frequency property. The magnetic permeability is reduced in a band of frequencies higher than frequencies at which ferrite starts resonance. Sufficient properties of the BALUN transformer cannot be obtained. There arises a problem that the core material cannot catch up with the high-frequency operation of the BALUN transformer.
To solve the problem, by lowering the magnetic permeability of the spinel ferrite and shifting the frequency at which resonance starts toward a high-frequency side to reduce a resonance loss, the BALUN transformer operable at a high frequency can be obtained. However, the ferrite has a logic of "Snake's limiting line". Therefore, it is impossible to raise the frequency at which the resonance starts up to a high-frequency band exceeding the limitation. A BALUN transformer cannot be realized which provides an excellent property in the high-frequency band.